Planetary gear train of automatic transmission for vehicles

ABSTRACT

A planetary gear train of claim of an automatic transmission for a vehicle may include an input shaft for receiving an engine torque, an output shaft for outputting a shifted torque, a first planetary gear set, a second planetary gear set, a third planetary gear set, a fourth planetary gear set, a first shaft, a second shaft, a third shaft, a fourth shaft, a fifth shaft selectively connectable with the first shaft and the second shaft, and directly connected with the input shaft, a sixth shaft selectively connectable with the second shaft, a seventh shaft selectively connectable with the second shaft, and an eighth shaft directly connected with the output shaft.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No. 10-2016-0032987, filed Mar. 18, 2016, the entire contents of which is incorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train of an automatic transmission of a vehicle that improves power delivery performance and reduces fuel consumption by achieving ten forward speed stages using a minimum number of constituent elements.

Description of Related Art

Research on realizing more shift-stages of an automatic transmission has been undertaken to achieve enhancement of fuel consumption and better drivability. Recent increases in oil prices are triggering stiff competition in enhancing fuel consumption of a vehicle.

In this sense, research on an engine has been undertaken to achieve weight reduction and to enhance fuel consumption by so-called downsizing and research on an automatic transmission has been performed to simultaneously provide better drivability and fuel consumption by achieving more shift stages.

However, in the automatic transmission, as the number of speed stages increase, the number of internal components (particularly, planetary gear sets) increase, and as a result, a length of the transmission increases. This may deteriorate installability and/or power flow efficiency and may increase production cost, and weight.

Therefore, in order to maximally enhance fuel consumption of an automatic transmission having more shift stages, it is important for better efficiency to be derived by a smaller number of parts.

In this respect, an eight-speed automatic transmission has been recently introduced, and a planetary gear train for an automatic transmission enabling more shift stages is under investigation.

An automatic transmission of eight or more shift-stages typically includes three to four planetary gear sets and five to six control elements (frictional elements), and may easily become lengthy, thereby deteriorating installability.

In this regard, disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements has been attempted. However, such an arrangement is not widely applicable, and using dog clutches may easily deteriorate shift-feel.

The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing a planetary gear train of an automatic transmission for a vehicle having advantages of obtaining shift-stages of at least ten forward speeds and at least one reverse speed by minimal number of parts, improving power delivery performance and fuel consumption by multi-stages of an automatic transmission, and improving driving stability of a vehicle by utilizing a low rotation speed of an engine.

According to various aspects of the present invention, a planetary gear train of an automatic transmission for a vehicle may include an input shaft for receiving an engine torque, an output shaft for outputting a shifted torque, a first planetary gear set including first, second, and third rotation elements, a second planetary gear set including fourth, fifth, and sixth rotation elements, a third planetary gear set including seventh, eighth, and ninth rotation elements, a fourth planetary gear set including tenth, eleventh, and twelfth rotation elements, a first shaft connected with the first rotation element, a second shaft connected with the second rotation element, a third shaft interconnecting the third rotation element, the eighth rotation element, and the eleventh rotation element, a fourth shaft connected with the fourth rotation element, a fifth shaft connected with the fifth rotation element, selectively connectable with the first shaft and the second shaft, and directly connected with the input shaft, a sixth shaft interconnecting the sixth rotation element, the seventh rotation element, and tenth rotation element, and selectively connectable with the second shaft, a seventh shaft connected with the ninth rotation element, and selectively connectable with the second shaft, and an eighth shaft connected with the twelfth rotation element, and directly connected with the output shaft.

The fourth shaft and the seventh shaft may each be selectively connectable with a transmission housing.

The first, second, and third rotation elements may be a first sun gear, a first planet carrier, and a first ring gear, the fourth, fifth, and sixth rotation elements may be a second sun gear, a second planet carrier, and a second ring gear, the seventh, eighth, and ninth rotation elements may be a third sun gear, a third planet carrier, and a third ring gear, and the tenth, eleventh, and twelfth rotation elements may be a fourth sun gear, a fourth planet carrier, and a fourth ring gear.

The planetary gear train may further include a first clutch selectively connecting the first shaft and the fifth shaft, a second clutch selectively connecting the second shaft and the fifth shaft, a third clutch selectively connecting the second shaft and the sixth shaft, a fourth clutch selectively connecting the second shaft and the seventh shaft, a first brake selectively connecting the fourth shaft and the transmission housing, and a second brake selectively connecting the seventh shaft and the transmission housing.

According to various aspects of the present invention, a planetary gear train of an automatic transmission for a vehicle may include an input shaft for receiving an engine torque, an output shaft for outputting a shifted torque, a first planetary gear set including first, second, and third rotation elements, a second planetary gear set including fourth, fifth, and sixth rotation elements, a third planetary gear set including seventh, eighth, and ninth rotation elements, and a fourth planetary gear set including tenth, eleventh, and twelfth rotation elements, in which the input shaft may be directly connected with the fifth rotation element, the output shaft may be directly connected with the twelfth rotation element, the third rotation element may be directly connected with the eighth rotation element and the eleventh rotation element, the fifth rotation element may be selectively connectable with the first rotation element and the second rotation element, the sixth rotation element may be selectively connectable with the second rotation element, the seventh rotation element and the tenth rotation element are directly connected with the sixth rotation element, and the ninth rotation element may be selectively connectable with the second rotation element.

The fourth rotation element and the ninth rotation element may each be selectively connectable with a transmission housing.

The planetary gear train may further include a first clutch selectively connecting the first rotation element and the fifth rotation element, a second clutch selectively connecting the second rotation element and the fifth rotation element, a third clutch selectively connecting the second rotation element and the sixth rotation element, a fourth clutch selectively connecting the second rotation element and the ninth rotation element, a first brake selectively connecting the fourth rotation element and the transmission housing, and a second brake selectively connecting the ninth rotation element and the transmission housing.

Speed stages implemented by selectively operating the first, second, third and fourth clutches and the first and second brakes may include a forward first speed stage, implemented by operating the first and third clutches, and the second brake, a forward second speed stage, implemented by operating the second and third clutches, and the second brake, a forward third speed stage, implemented by operating the second clutch, and the first and second brakes, a forward fourth speed stage, implemented by operating the first and second clutches, and the second brake, a forward fifth speed stage, implemented by operating the first and second clutches, and the first brake, a forward sixth speed stage, implemented by operating the first, second fourth clutches, a forward seventh speed stage, implemented by operating the second and fourth clutches, and the first brake, a forward eighth speed stage, implemented by operating the first and fourth clutches, and the first brake, a forward ninth speed stage, implemented by operating the third and fourth clutches, and the first brake, a forward tenth speed stage, implemented by operating the first and third clutches, and the first brake, and a reverse-speed stage, implemented by operating the first and fourth clutches, and the second brake.

According to various embodiments of the present invention, shift-stages of at least ten forward speeds and at least one reverse speed are realized by combination of four planetary gear sets and six control elements.

In addition, a planetary gear train according to various embodiments of the present invention substantially improves driving stability by realizing shift-stages appropriate for rotation speed of an engine due to multiple speed-stages of an automatic transmission.

Furthermore, a planetary gear train according to various of the present invention maximizes engine driving efficiency by multiple speed-stages of an automatic transmission, and improves power delivery performance and fuel consumption.

It is understood that the term “vehicle” or “vehicular” or other similar terms as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g., fuel derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example, both gasoline-powered and electric-powered vehicles.

The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a planetary gear train according to various embodiments of the present invention.

FIG. 2 is an operational chart for respective control elements at respective shift-stages in the planetary gear train according to various embodiments of the present invention.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

FIG. 1 is a schematic diagram of a planetary gear train according to various embodiments of the present invention.

Referring to FIG. 1, the planetary gear train according to various embodiments of the present invention includes first, second, third, and fourth planetary gear set PG1, PG2, PG3, and PG4, arranged on a same axis, an input shaft IS, an output shaft OS, eight shafts TM1 to TM8 interconnecting rotation elements of the first, second, third, and fourth planetary gear set PG1, PG2, PG3, and PG4, four clutches C1 to C4 and two brakes B1 and B2 as control elements, and a transmission housing H.

Torque input from the input shaft IS is shifted by cooperative operation of the first, second, third, and fourth planetary gear sets PG1, PG2, PG3, and PG4, and then output through the output shaft OS.

The planetary gear sets are arranged in the order of first, second, third, and fourth planetary gear sets PG1, PG2, PG3, and PG4, from an engine side.

The input shaft IS is an input member and the torque from a crankshaft of an engine is input into the input shaft IS, after being torque-converted through a torque converter.

The output shaft OS is an output member, and being arranged on a same axis with the input shaft IS, delivers a shifted driving torque to a drive shaft through a differential apparatus.

The first planetary gear set PG1 is a single pinion planetary gear set, and includes a first planet carrier PC1 that supports first pinion gear P1 externally engaged with the first sun gear S1, and a first ring gear R1 that is internally engaged with the first pinion gear P1. The first sun gear S1 acts as a first rotation element N1, the first planet carrier PC1 acts as a second rotation element N2, and the first ring gear R1 acts as a third rotation element N3.

The second planetary gear set PG2 is a single pinion planetary gear set, and includes a second planet carrier PC2 that supports second pinion gear P2 externally engaged with the second sun gear S2, and a second ring gear R2 that is internally engaged with the second pinion gear P2. The second sun gear S2 acts as a fourth rotation element N4, the second planet carrier PC2 acts as a fifth rotation element N4, and the second ring gear R2 acts as a sixth rotation element N6.

The third planetary gear set PG3 is a single pinion planetary gear set, and includes a third planet carrier PC3 that supports third pinion gear P3 externally engaged with the third sun gear S3, and a third ring gear R3 that is internally engaged with the third pinion gear P3. The third sun gear S3 acts as a seventh rotation element N7, the third planet carrier PC3 acts as an eighth rotation element N8, and the third ring gear R3 acts as a ninth rotation element N9.

The fourth planetary gear set PG4 is a single pinion planetary gear set, and includes a fourth planet carrier PC4 that supports fourth pinion gear P4 externally engaged with the fourth sun gear S4, and a fourth ring gear R4 that is internally engaged with the fourth pinion gear P4. The fourth sun gear S4 acts as a tenth rotation element N10, the fourth planet carrier PC4 acts as a eleventh rotation element N11, and the fourth ring gear R4 acts as a twelfth rotation element N12.

In the first, second, third, and fourth planetary gear set PG1, PG2, PG3, and PG4, the third rotation element N3 is directly connected with the eighth rotation element N8 and the eleventh rotation element N11, and the sixth rotation element N6 is directly connected with the seventh rotation element N7 and the tenth rotation element N10, by eight shafts TM1 to TM8.

The eight shafts TM1 to TM8 are hereinafter described in detail.

Each of the eight shafts TM1 to TM8 may be a rotation member that directly interconnects the input and output shafts and rotation elements of the planetary gear sets PG1, PG2, PG3, and PG4, or may be a fixed member fixed to the transmission housing H.

The first shaft TM1 is connected with the first rotation element N1 (the first sun gear S1).

The second shaft TM2 is connected the second rotation element N2 (the first planet carrier PC1).

The third shaft TM3 directly connects the third rotation element N3 (the first ring gear R1), the eighth rotation element N8 (the third planet carrier PC3), and the eleventh rotation element N11 (the fourth planet carrier PC4).

The fourth shaft TM4 is connected with the fourth rotation element N4 (the second sun gear S2), and is selectively connectable with the transmission housing H.

The fifth TM5 is connected the fifth rotation element N5 (the second planet carrier PC2), is selectively connectable with the first shaft TM1 and the second shaft TM2, and directly connected with the input shaft IS, thereby continuously acting as an input element.

The sixth shaft TM6 connects the sixth rotation element N6 (the second ring gear R2), the seventh rotation element N7 (the third sun gear S3), and the tenth rotation element N10 (the fourth sun gear S4), and is selectively connectable with the second shaft TM2.

The seventh shaft TM7 is connected with the ninth rotation element N9 (the third ring gear R3), is selectively connectable with the second shaft TM2, and is selectively connectable with the transmission housing H, thereby selectively acting as a fixed element.

The eighth shaft TM8 is connected with the twelfth rotation element N12 (the fourth ring gear R4), and is directly connected with the output shaft OS, thereby continuously acting as an output element.

The eight shafts TM1 to TM8, the input shaft IS, and the output shaft OS may be selectively interconnected with one another by control elements of four clutches C1, C2, C3, and C4.

The eight shafts TM1 to TM8 may be selectively connectable with the transmission housing H, by control elements of two brakes B1 and B2.

The four clutches C1 to C4 and the two brakes B1 and B2 are arranged as follows.

The first clutch C1 is arranged between the first shaft TM1 and the fifth shaft TM5, and selectively connects the first shaft TM1 and the fifth shaft TM5 connected with the output shaft OS thereby controlling power delivery therebetween.

The second clutch C2 is arranged between the second shaft TM2 and the fifth shaft TM5, and selectively connects the second shaft TM2 and the fifth shaft TM5 connected with the output shaft OS thereby controlling power delivery therebetween.

The third clutch C3 is arranged between the second shaft TM2 and the sixth shaft TM6, and selectively connects the second shaft TM2 and sixth shaft TM6 connected with the output shaft OS thereby controlling power delivery therebetween.

The fourth clutch C4 is arranged between the second shaft TM2 and the seventh shaft TM7, and selectively connects the second shaft TM2 and the seventh shaft TM7 connected with the output shaft OS thereby controlling power delivery therebetween.

The first brake B1 is arranged between the fourth shaft TM4 and the transmission housing H, and selectively connects the fourth shaft TM4 to the transmission housing H.

The second brake B2 is arranged between the seventh shaft TM7 and the transmission housing H, and selectively connects the seventh shaft TM7 to the transmission housing H.

The respective control elements of the first, second, third, and fourth clutches C1, C2, C3, and C4 and the first and second brakes B1 and B2 may be realized as multi-plate hydraulic pressure friction devices that are frictionally engaged by hydraulic pressure.

FIG. 2 is an operational chart for respective control elements at respective shift-stages in a planetary gear train according to various embodiments of the present invention.

Referring to FIG. 2, the planetary gear train according to various embodiments of the present invention realizes ten forward speeds and one reverse speed by operating three control elements among the first, second, third, and fourth clutches C1, C2, C3, and C4 and the first and second brakes B1 and B2 at respective shift-stages.

In the forward first speed shift-stage D1, the first and third clutches C1 and C3 and the second brake B2 are operated.

As a result, the first shaft TM1 is connected with the fifth shaft TM5 by the operation of the first clutch C1, and the second shaft TM2 is connected with the sixth shaft TM6 by the operation of the third clutch C3. In this state, the torque of the input shaft IS is input to the fifth shaft TM5.

In addition, the seventh shaft TM7 acts as a fixed element by the operation of the second brake B2, thereby realizing the forward first speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the eighth shaft TM8.

In the forward second speed shift-stage D2, the second and third clutches C2 and C3 and the second brake B2 are operated.

As a result, the second shaft TM2 is connected with the fifth shaft TM5 by operation of the second clutch C2, and the second shaft TM2 is connected with the sixth shaft TM6 by the operation of the third clutch C3. In this state, the torque of the input shaft IS is input to the fifth shaft TM5.

In addition, the seventh shaft TM7 acts as a fixed element by the operation of the second brake B2, thereby realizing the forward second speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the eighth shaft TM8.

In the forward third speed shift-stage D3, the second clutch C2 and the first and second brakes B1 and B2 are operated.

As a result, the second shaft TM2 is connected with the fifth shaft TM5 by operation of the second clutch C2. In this state, the torque of the input shaft IS is input to the fifth shaft TM5.

In addition, the fourth and seventh shafts TM4 and TM7 act as fixed elements by the operation of the first and second brakes B1 and B2, thereby realizing the forward third speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the eighth shaft TM8.

In the forward fourth speed shift-stage D4, the first and second clutches C1 and C2 and the second brake B2 are operated.

As a result, the first shaft TM1 is connected with the fifth shaft TM5 by the operation of the first clutch C1, and the second shaft TM2 is connected with the fifth shaft TM5 by operation of the second clutch C2. In this state, the torque of the input shaft IS is input to the fifth shaft TM5.

In addition, the seventh shaft TM7 acts as a fixed element by the operation of the second brake B2, thereby realizing the forward fourth speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the eighth shaft TM8.

In the forward fifth speed shift-stage D5, the first and second clutches C1 and C2 and the first brake B1 are operated.

As a result, the first shaft TM1 is connected with the fifth shaft TM5 by the operation of the first clutch C1, and the second shaft TM2 is connected with the fifth shaft TM5 by operation of the second clutch C2. In this state, the torque of the input shaft IS is input to the fifth shaft TM5.

In addition, the fourth shaft TM4 acts as a fixed element by the operation of the first brake B1, thereby realizing the forward fifth speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the eighth shaft TM8.

In the forward sixth speed shift-stage D6, the first, second, and fourth clutches C1, C2, and C4 are operated.

As a result, the first shaft TM1 is connected with the fifth shaft TM5 by the operation of the first clutch C1, the second shaft TM2 is connected with the fifth shaft TM5 by operation of the second clutch C2, and the second shaft TM2 is connected with the seventh shaft TM7 by operation of the fourth clutch C4. In this state, the torque of the input shaft IS is input to the fifth shaft TM5.

In this case, entire planetary gear sets PG1, PG2, PG3, and PG4 integrally rotate, and a torque is output as input, thereby forming the forward sixth speed and outputting the input torque to the output shaft OS connected with the eighth shaft TM8.

In the forward seventh speed shift-stage D7, the second and fourth clutches C2 and C4 and the first brake B1 are operated.

As a result, the second shaft TM2 is connected with the fifth shaft TM5 by operation of the second clutch C2, and the second shaft TM2 is connected with the seventh shaft TM7 by operation of the fourth clutch C4. In this state, the torque of the input shaft IS is input to the fifth shaft TM5.

In addition, the fourth shaft TM4 acts as a fixed element by the operation of the first brake B1, thereby realizing the forward seventh speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the eighth shaft TM8.

In the forward eighth speed shift-stage D8, the first and fourth clutches C1 and C4 and the first brake B1 are operated.

As a result, the first shaft TM1 is connected with the fifth shaft TM5 by the operation of the first clutch C1, and the second shaft TM2 is connected with the seventh shaft TM7 by operation of the fourth clutch C4. In this state, the torque of the input shaft IS is input to the fifth shaft TM5.

In addition, the fourth shaft TM4 acts as a fixed element by the operation of the first brake B1, thereby realizing the forward eighth speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the eighth shaft TM8.

In the forward ninth speed shift-stage D9, the third and fourth clutches C3 and C4 and the first brake B1 are operated.

As a result, the second shaft TM2 is connected with the sixth shaft TM6 by the operation of the third clutch C3, and the second shaft TM2 is connected with the seventh shaft TM7 by operation of the fourth clutch C4. In this state, the torque of the input shaft IS is input to the fifth shaft TM5.

In addition, the fourth shaft TM4 acts as a fixed element by the operation of the first brake B1, thereby realizing the forward ninth speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the eighth shaft TM8.

In the forward tenth speed shift-stage D10, the first and third clutches C1 and C3 and the first brake B1 are operated.

As a result, the first shaft TM1 is connected with the fifth shaft TM5 by the operation of the first clutch C1, and the second shaft TM2 is connected with the sixth shaft TM6 by the operation of the third clutch C3. In this state, the torque of the input shaft IS is input to the fifth shaft TM5.

In addition, the fourth shaft TM4 acts as a fixed element by the operation of the first brake B1, thereby realizing the forward tenth speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the eighth shaft TM8.

In the reverse speed REV, the first and fourth clutches C1 and C4, and the second brake B2 are operated.

As a result, the first shaft TM1 is connected with the fifth shaft TM5 by the operation of the first clutch C1, and the second shaft TM2 is connected with the seventh shaft TM7 by operation of the fourth clutch C4. In this state, the torque of the input shaft IS is input to the fifth shaft TM5.

In addition, the seventh shaft TM7 acts as a fixed element by the operation of the second brake B2, thereby realizing the reverse speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the eighth shaft TM8.

As described above, a planetary gear train according to various embodiments of the present invention realizes at least ten forward speeds and at least one reverse speed by operating four planetary gear sets PG1, PG2, PG3, and PG4 by controlling the four clutches C1, C2, C3, and C4 and the two brakes B1 and B2.

In addition, a planetary gear train according to various embodiments of the present invention realizes shift stages appropriate for rotation speed of an engine due to multiple speed-stages of an automatic transmission and improves driving stability of a vehicle by utilizing a low rotation speed of an engine.

Furthermore, a planetary gear train according to various embodiments of the present invention maximizes engine driving efficiency by multiple speed-stages of an automatic transmission, and improves power delivery performance and fuel consumption.

The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents. 

What is claimed is:
 1. A planetary gear train of an automatic transmission for a vehicle, comprising: an input shaft for receiving an engine torque; an output shaft for outputting a shifted torque; a first planetary gear set including a first rotation element, a second rotation element, and a third rotation element; a second planetary gear set including a fourth rotation element, a fifth rotation element, and a sixth rotation element; a third planetary gear set including a seventh rotation element, an eighth rotation element, and a ninth rotation element; a fourth planetary gear set including a tenth rotation element, an eleventh rotation element, and a twelfth rotation element; a first shaft connected with the first rotation element; a second shaft connected with the second rotation element; a third shaft interconnecting the third rotation element, the eighth rotation element, and the eleventh rotation element; a fourth shaft connected with the fourth rotation element; a fifth shaft connected with the fifth rotation element, selectively connectable with the first shaft and the second shaft, and directly connected with the input shaft; a sixth shaft interconnecting the sixth rotation element, the seventh rotation element, and tenth rotation element, and selectively connectable with the second shaft; a seventh shaft connected with the ninth rotation element, and selectively connectable with the second shaft; and an eighth shaft connected with the twelfth rotation element, and directly connected with the output shaft.
 2. The planetary gear train of claim 1, wherein the fourth shaft and the seventh shaft are each selectively connectable with a transmission housing.
 3. The planetary gear train of claim 1, wherein the first rotation element, the second rotation element, and the third rotation element comprise a first sun gear, a first planet carrier, and a first ring gear; the fourth rotation element, the fifth rotation element, and the sixth rotation element comprise a second sun gear, a second planet carrier, and a second ring gear; the seventh rotation element, the eighth rotation element, and the ninth rotation element comprise a third sun gear, a third planet carrier, and a third ring gear; and the tenth rotation element, the eleventh rotation element, and the twelfth rotation element comprise a fourth sun gear, a fourth planet carrier, and a fourth ring gear.
 4. The planetary gear train of claim 2, further comprising: a first clutch selectively connecting the first shaft and the fifth shaft; a second clutch selectively connecting the second shaft and the fifth shaft; a third clutch selectively connecting the second shaft and the sixth shaft; a fourth clutch selectively connecting the second shaft and the seventh shaft; a first brake selectively connecting the fourth shaft and the transmission housing; and a second brake selectively connecting the seventh shaft and the transmission housing.
 5. A planetary gear train of an automatic transmission for a vehicle, comprising: an input shaft for receiving an engine torque; an output shaft for outputting a shifted torque; a first planetary gear set including a first rotation element, a second rotation element, and a third rotation element; a second planetary gear set including a fourth rotation element, a fifth rotation element, and a sixth rotation element; a third planetary gear set including a seventh rotation element, an eighth rotation element, and a ninth rotation element; and a fourth planetary gear set including a tenth rotation element, an eleventh rotation element, and a twelfth rotation element, wherein the input shaft is directly connected with the fifth rotation element, the output shaft is directly connected with the twelfth rotation element, the third rotation element is directly connected with the eighth rotation element and the eleventh rotation element, the fifth rotation element is selectively connectable with the first rotation element and the second rotation element, the sixth rotation element is selectively connectable with the second rotation element, the seventh rotation element and the tenth rotation element are directly connected with the sixth rotation element, and the ninth rotation element is selectively connectable with the second rotation element.
 6. The planetary gear train of claim 5, wherein the fourth rotation element and the ninth rotation element are each selectively connectable with a transmission housing.
 7. The planetary gear train of claim 5, wherein the first rotation element, the second rotation element, and the third rotation element comprise a first sun gear, a first planet carrier, and a first ring gear; the fourth rotation element, the fifth rotation element, and the sixth rotation element comprise a second sun gear, a second planet carrier, and a second ring gear; the seventh rotation element, the eighth rotation element, and the ninth rotation element comprise a third sun gear, a third planet carrier, and a third ring gear; and the tenth rotation element, the eleventh rotation element, and the twelfth rotation element comprise a fourth sun gear, a fourth planet carrier, and a fourth ring gear.
 8. The planetary gear train of claim 6, further comprising: a first clutch selectively connecting the first rotation element and the fifth rotation element; a second clutch selectively connecting the second rotation element and the fifth rotation element; a third clutch selectively connecting the second rotation element and the sixth rotation element; a fourth clutch selectively connecting the second rotation element and the ninth rotation element; a first brake selectively connecting the fourth rotation element and the transmission housing; and a second brake selectively connecting the ninth rotation element and the transmission housing.
 9. The planetary gear train of claim 8, wherein speed stages implemented by selectively operating the first, second, third and fourth clutches and the first and second brakes include: a first forward speed stage, implemented by operating the first and third clutches, and the second brake; a second forward speed stage, implemented by operating the second and third clutches, and the second brake; a third forward speed stage, implemented by operating the second clutch, and the first and second brakes; a fourth forward speed stage, implemented by operating the first and second clutches, and the second brake; a fifth forward speed stage, implemented by operating the first and second clutches, and the first brake; a sixth forward speed stage, implemented by operating the first, second fourth clutches; a seventh forward speed stage, implemented by operating the second and fourth clutches, and the first brake; an eighth forward speed stage, implemented by operating the first and fourth clutches, and the first brake; a ninth forward speed stage, implemented by operating the third and fourth clutches, and the first brake; a forward tenth speed stage, implemented by operating the first and third clutches, and the first brake; and a reverse-speed stage, implemented by operating the first and fourth clutches, and the second brake. 